Lovastatin and derivatives derived therefrom (e.g. Simvastatin) are potent HMG-CoA reductase inhibitors. Lovastatin is produced in fermentations with Aspergillus terreus as described in U.S. Pat. No. 4,231,938 by Monaghan et al. A 4a,5-dihydrolovastatin (I) is co-produced with lovastatin in considerably ##STR1## lower yield as described in U.S. Pat. No. 4,294,846 by Albers-Schonberg et al. However, this naturally occurring 4a,5-dihydromevinolin appears to be slightly more active than lovastatin as an HMB-CoA reductase inhibitor. Efforts to convert the more abundant lovastatin to its more active 4a,5-dihydro derivative have been made by catalytic reduction as reported in published EP application No. 0033537 but no evidence of the desired trans-fused 4a,5-dihydrolovastatin was found.
Kuo et al. (U.S. Pat. No. 4,490,546 and J. Org. Chem. 48, 1991 (1983)) have disclosed a process for hydrogenating the 4a,5 double bond of lovastatin. However this process requires 5 separate steps leading to the natural trans isomer in about 10 percent yield. Moreover the desired product is contaminated with the cis-fused dihydro derivative.
The use of a ligating group, such as OH, on an olefinic substrate is known to direct the attack of a hydrogenation catalyst such as [Ir(COD)PCy.sub.3 (pyr)]PF.sub.6 from the face of the moleule containing the directing group. However, there are no reports of selective hydrogenation of a conjugated diene, with catalysts of this type.